A conventional sleeve antenna comprises a radiation element having an electrical length of one quarter wavelength, a sleeve having an electrical length of one quarter wavelength, and a coaxial cable for feeding an electric power to the radiation element, wherein an outer conductor of the coaxial cable is connected to the sleeve, while an inner conductor of the coaxial cable is extended through the sleeve to be connected to the radiation element.
A conventional inverted type coaxial dipole antenna is structured such that a central conductor of a coaxial cable is connected via a feeding line to a sleeve, wherein the feeding line is extended through a slot which is formed through an outer tube.
A conventional flat antenna comprises a flat substrate, on a first surface of which a micro-strip of a thin conductive film is formed, and on a second surface of which a dipole antenna element and a feeding slot are formed.
In the conventional antennas as simply explained above, however, there are disadvantages as set out below.
The conventional sleeve antenna and inverted type coaxial dipole antenna involve complicated fabrication and adjustment because the feeding coaxial cable is connected to the sleeve. This is causative of instable quality.
On the other hand, the flat antenna can solve the above problems involved in the conventional sleeve antenna and inverted type coaxial dipole antenna. It, however, cannot be diversified because a feeding slot is provided on the ground plane. The diversification refers to combining of a plurality of antennas to enhance the transmitting/receiving capacity.
Japanese Patent Laid-Open No. 59-97207 discloses a diversity antenna which comprises two sleeve antennas vertically arranged to provide diversity effect.
The diversity antenna, however, is disadvantageously heavy and difficult to handle. Further, in the individual sleeve antennas, there are many steps requiring handwork, such as fabrication and adjustment for connecting the feeding coaxial cable to the sleeve, which is causative of instable quality and in addition makes it difficult to reduce the cost.